std::regex_iterator is a read-only ForwardIterator that accesses the individual matches of a regular expression within the underlying character sequence.

On construction, and on every increment, it calls std::regex_search and remembers the result (that is, saves a copy of the value std::match_results<BidirIt>). The first object may be read when the iterator is constructed or when the first dereferencing is done. Otherwise, dereferencing only returns a copy of the most recently obtained regex match.

The default-constructed std::regex_iterator is the end-of-sequence iterator. When a valid std::regex_iterator is incremented after reaching the last match (std::regex_search returns false), it becomes equal to the end-of-sequence iterator. Dereferencing or incrementing it further invokes undefined behavior.

A typical implementation of std::regex_iterator holds the begin and the end iterators for the underlying sequence (two instances of BidirIt), a pointer to the regular expression (const regex_type*), the match flags (std::regex_constants::match_flag_type), and the current match (std::match_results<BidirIt>).

It is the programmer's responsibility to ensure that the std::basic_regex object passed to the iterator's constructor outlives the iterator. Because the iterator stores a pointer to the regex, incrementing the iterator after the regex was destroyed accesses a dangling pointer.

If the part of the regular expression that matched is just an assertion (^, $, \b, \B), the match stored in the iterator is a zero-length match, that is, match[0].first== match[0].second.